Discovery and development of drugs that are orally bioavailable, in addition to being potent and selective, presents a major challenge to the pharmaceutical industry. Compounds that can easily partition into the lipid bilayer of the cell membrane are generally easily absorbed (transcellular route). In contrast, hydrophilic compounds are substantially unable to cross the cell membrane. Their passage across the epithelium via the intercellular route (paracellular route) is also severely restricted by the presence of intercellular junctions, known as tight junctions. Thus, the intestinal epithelium poses a significant barrier to absorption of orally administered hydrophilic therapeutic agents.
The tight junctions are highly specialized multi-protein complexes that are composed of both transmembrane and cytsolic proteins. Denkar, B. M. and Nigam, S. K. (1998) Am. J. Physiol. 274:F1-F9; Fanning, A. S., et al., J. Soc. Nephrol. 10:1337-1345 (1999). The opening of the tight junctions appears to be regulated by complex intracellular signaling mechanisms. Anderson, J. M., et al., J. Cell Biol. 106:1141-1149 (1998); Sakakibara, A., et al., J. Cell. Biol. 137:1393-1401 (1997). Many signaling pathways have been implicated in the regulation of paracellular permeability (e.g., tyrosine kinases, calcium, protein kinase C (PKC)). Anderson, J. M., and Van Italle, C. M., Am. J. Phys. 269:G467-G475 (1995); Collares-Buzato, C. B., et al., Eur. J. Cell Biol. 76:85-92 (1998); Mullin, J. M., et al., Am. J. Physiol. 275:C544-C554 (1998); Tai, Y. H., et al., J. Membr. Biol. 149:71-79 (1996). However, the molecular mechanisms associated with this regulation have not been thoroughly elucidated.
To date, several absorption-enhancing agents have been identified that improve the absorption of drug candidates across intestinal epithelium by modulation of tight junctions. However, most of these agents lack potency and selectivity for action on the tight junctions and thus, must be administered in amounts that produce millimolar (mM) in vivo concentrations. Such concentrations can lead to cytotoxicity and undesirable side effects. Additionally, their mechanism of action is unknown or poorly understood, and frequently, these agents act via multiple mechanisms. Hence, pharmaceutically acceptable absorption enhancers are not available for clinical use.
What is needed, then, is an absorption-enhancing agent that exhibits potency and selectivity for the enhancement of paracellular permeability. Further characterization of tight junction regulation and identification of agents that influence tight junction regulation thus represent long felt and continuing needs in the art.